The invention concerns a process for the production of multi-layer metal composite tubes, wherein a metal strip is shaped to form a tube and welded along its two longitudinal edges, and wherein a plastic inner layer is applied by extrusion at the inside of the metal tube and a plastic outer layer is applied by extrusion at the outside of the metal tube.
Such processes are described in DE 30 16 134 A1, WO 88/03084 A1, EP 0 353 977 A2, EP 0 581 208 A1 or EP 0 920 972 A1. All those known processes involve shaping the metal strip to form a tube, with the two longitudinal edges of the metal strip which has been converted into tube form overlapping. The mutually overlapping longitudinal edges are welded. After the operation of welding the mutually overlapping longitudinal edges, the plastic inner layer is then fixedly applied at the inside of the metal tube and the plastic outer layer is fixedly applied at the outside of the metal tube. For that purpose, a respective bonding agent is extruded with each of the plastic inner layer and the plastic outer layer. The plastic materials used for the inner and outer layers are cross-linked or non-cross-linked plastic materials.
Multi-layer metal composite tubes with welded, mutually overlapping longitudinal edges suffer from the deficiency, in particular in the case of relatively large diametral dimensions with which the wall thickness or gauge of the metal tube correspondingly increases, that the mutually overlapping longitudinal edges become noticeable as a consequence of the wall thickness involvedxe2x80x94which is insignificant in the case of composite tubes of small diameter and correspondingly thin metal strips or tubes.
The German journal xe2x80x9cWerkstatt+Montagepraxisxe2x80x9d, Dipl.xe2x80x94Ing. Horst Rahn: xe2x80x9cRohrleitungen aus Kunststoff und Metall in der Sanitxc3xa4rxe2x80x94und Heizungstechnikxe2x80x9d, pages 30 to 36, September/October 1993, besides that above-mentioned process with mutually overlapping longitudinal edges of the metal strip which is shaped to form the tube, also describes what is known as the Swiss process which comprises firstly producing an inner plastic tube on which is arranged a butt-welded metal or aluminium tube. With that process, it is necessary for the tube which is butt-welded at its two longitudinal edges to be of an inside diameter which is slightly larger than the outside diameter of the plastic inner tube, in order to be able to push the metal tube onto the plastic inner tube. After the metal tube has been pushed onto the plastic inner tube, it is necessary for the metal tube to be calibrated down to the size of the plastic inner tube. Thereafter, the metal or aluminium tube is coated by extrusion with an outer plastic sheathing. In order reliably to be able to implement butt-welding however, it is necessary for the metal tube to be of a given minimum wall thickness. That is frequently more than twice as thick as the minimum wall thickness when welding the mutually overlapping longitudinal edges of a metal strip which has been shaped to form a tube.
A process for the production of a composite tube which comprises at least two layers, wherein at least one layer comprises plastic material and one layer comprises metal, is also described for example in DE 43 10 272 A1. It is proposed therein that the longitudinal edges of the metal strip which is shaped to form a tube are crimped or flanged up in a profiling tool and the flanged longitudinal edges are then welded with a longitudinal seam welding apparatus. That process is relatively complicated and is scarcely suitable or is not at all suitable for inexpensive mass production.
DE 195 36 698 C1 and DE 44 04 492 A1 disclose processes for the production of multi-layer metal composite tubes, wherein a metal strip is shaped to form a tube and is welded along its two longitudinal edges. In that procedure, in accordance with DE 195 36 698 C1, a prefabricated plastic inner tube is applied substantially flush to the metal tube. In that case therefore a prefabricated plastic inner tube is combined with a metal strip which is shaped to form a tube. That represents a considerable amount of apparatus complication and expenditure. In addition, in that procedure, prior to the longitudinal seam welding operation, at the edges which are opposite to each other at the longitudinal seam, the metal tube is profiled outwardly inclinedly or arcuately or in a step configuration, so that an air gap remains between the edges and the outside surface of the prefabricated plastic inner tube. That air gap is required there in order to avoid the prefabricated plastic inner tube being adversely affected during the longitudinal seam welding operation. That profiling of the longitudinal edges of the metal strip also represents an expenditure which cannot be disregarded. Insofar as that longitudinal profiling does not in any case disappear in the longitudinal seam welding operation, that known process also requires the profiled longitudinal edges to be shaped to bring them back into the plane of the prefabricated outer tube. That also represents a complication and expenditure which is not to be disregarded.
DE 44 04 492 A1 provides combining a prefabricated plastic inner tube with a metal tube, wherein a metal foil with mutually overlapping edges is applied to the prefabricated plastic inner tube. The mutually overlapping edges are welded by means of a laser. This known process of the last-mentioned kind therefore not only involves using a prefabricated plastic inner tube, but in addition it involves producing only a metal tube with mutually overlapping longitudinal edges and welding same along the longitudinal edges.
It is an object of the present invention to utilise the advantages of a metal tube with mutually overlapping welded longitudinal edges, with a small metal tube wall thickness and with corresponding flexibility or reduced flexural stiffness, without an overlap making itself annoyingly apparent.
In accordance with the invention, the foregoing object is attained by the process for the production of multi-layer metal composite tubes, wherein the metal strip is shaped to form a tube in such a way that the two longitudinal edges of the metal strip which is shaped to form the tube adjoin each other in butting relationship without a change in diameter and are butt-welded, wherein immediately after the longitudinal butt-welding of the tube the plastic inner layer and the plastic outer layer are extruded onto the tube. In that respect it has proven to be advantageous if the two longitudinal edges are welded by means of a laser welding apparatus. Another possible option provides that the two longitudinal edges are welded by means of an electrode welding apparatus, by means of an induction welding apparatus or by means of a flame welding apparatus.
The fact that, with the process according to the invention, the two longitudinal edges of the metal tube are butt-welded together, affords the advantage that the longitudinal edges of the metal strip which is shaped to form the tube do not make themselves in any way disturbingly noticeable.